Process for coordinated operation of diaphragm and mercury cathode electrolytic cells



-. a Si 637 efgaee fi e fi e s weiiiefia it Eli, rt... Patented Aug. 2%,1962 l 2 3 637 stantially complete removal of the heavy metals, such asPRQCES FGR CGORDINATED OPERATE-9N F DIAPHRAGM AND MERCURY CATHGDE ELEC-TROLYTIC CELLS Richard H. Judice, Houston, Ten, and Henry R. Wiesner,

South Euclid, Ohio, assignors to Diamond Alkali 01mpany, Cleveland,Ghio, a corporation of Delaware Filed June 10, 1959, Ser. No. 819, 34 9Claims. (Cl. 2ll498) This invention relates to a method for supplyingalkali metal halide brines for use in industrial processes and, moreparticularly, relates .to a method of supplying alkali metal halidebrines for use in electrolytic cells for the production of chlorine andalkalies, and still more particularly, relates to a method for supplyingalkali metal halide brines to such electrolytic cell systems whichconsists of both diaphragm cells and mercury cells.

Chlorine, for the most part, is produced commercially by theelectrolysis of an alkali metal chloride brine, such as a sodiumchloride brine, with the corresponding alkali metal hydroxide also beingproduced as a product of the electrolysis. In general, two differenttypes of electrolytic cells are used to effect this electrolysis, i.e.,diaphragm cells and mercury cells. Although in both types of cells,sodium chloride brine is electrolyzed using a carbon or graphite anodeand chlorine and sodium hydroxide are recovered as the ultimateproducts, it is at this point that the similarity of the two types ofcells ceases.

In the diaphragm cell, the cathode is generally of metal, such as iron,and is separated from the anode by a permeable diaphragm, generally ofasbestos. Additionally, in the diaphragm cell, the sodium hydroxide isrecovered at the cathode in admixture with sodium chloride and sodiumsulfate, which mixture is referred to generally as the cell liquor. Thiscatholyte or cell liquor normally contains about 11% sodium hydroxideand 14 to 15% sodium chloride, which sodium chloride must be separatedfrom the sodium hydroxide. This separation is eiieeted by theevaporation of the cell liquor to a concentration of about 50% sodiumhydroxide, at which strength the sodium chloride content ranges fromabout 0.8 to 2.0% depending upon the temperature of the sodium hydroxidesolution. The above percentages as well as those noted elsewhere hereinare percentages by weight, as is customary in the electrolyticchlorinealkali industry with reference to percentages of components ofsolutions containing caustic soda.

In contrast, in the mercury cell, there is no diaphragm and the cathodeis a moving film of mercury which passes through the cell. Additionally,the sodium produced by the electrolysis of the brine forms an amalgamwith the mercury, from which amalgam sodium hydroxide is recovered inconcentrations ranging up to 70% without the necessity for evaporation.In addition to these differences in the component parts of the diaphragmand mercury cells, as well as the difierence in the form in which theproduct sodium hydroxide is initially recovered, these types ofelectrolytic cells also differ as to the purity of the alkali metalhalide brine feed required by each.

In the diaphragm cell, substantially complete removal of the calcium andmagnesium impurities in the brine feed is essential in order .to preventblockage of the diaphragms. Additionally, for efficient operation of thecell, it is desirable to maintain the sulfate content of the brine belowabout 5.0 g./l. Moreover, in diaphragm cell operation, it is necessaryto provide a means for purging the sulfates from the system. On theother hand, for mercury cells, calcium impurities in the brine feed arenot considered to be critical but magnesium impurities are especiallybad so that substantially complete removal of these latter impurities isnecessary. Additionally, subiron, nickel, vanadium, chromium andmolybdenum, as well as aluminum, is essential inasmuch as these metalscause a break-down of the amalgam, thereby tending to cause a hydrogendischarge which leads to a dangerous concentration of hydrogen in thechlorine. As with the diaphragm cells, a sulfate content of only about10 g./l. can be tolerated so that purging of the sulfate is likewisenecessary. Moreover, in the operation of a mercury cell, where the brineis to be treated to remove impurities, it is necessary to dechlorinatethe brine which has passed through the cell before treatment,resaturation and recycling of the brine to the cell.

Generally speaking, the installation costs of a mercury cell areslightly greater than those for a diaphragm cell. However, the mercurycell has an advantage in that the sodium hydroxide produced has a verylow impurity content and is, thus, suitable for use in making rayonwithout further purification. In contrast, the sodium hydroxide from adiaphragm cell, being produced in admixture with at least about equalportions of sodium chloride, is recovered from the evaporator with ashigh as 1% sodium chloride contained therein. To be suitable for manyuses, such as for making rayon, this sodium hydroxide must be furtherpurified, which purification increases the cost of the diaphragm cellsodium hydroxide to at least that of the mercury cell.

Inasmuch as all consumers do not require sodium hydroxide of mercurycell quality nor are they willing to pay a premium price for such aproduct, it is apparent that there are definite advantages to beobtained from an operation combining both mercury cells and diaphragmcells. These advantages stem chiefly from the fact that by such anoperation it is possible to supply sodium hydroxide of either mercurycell quality or diaphragm cell quality, depending upon which is desired,without the additional expanse incurred in purifying diaphragm cellsodium hydroxide to obtain caustic of useable quality.

In the coordinated operation of both diaphragm cells and mercury cells,the solid salt recovered from the diaphragm cells, by the evaporation ofthe cell liquor or catholyte, is used to saturate the circulating brineof the mercury cells, which brine is depleted in sodium chlorideconcentration in each pass through the mercury cell. In this manner, thesalt recovered from the diaphragm cell can be utilized and there is aready supply of solid salt for use in the mercury cells. However, asadvantageous as such an operation might at first appear, difiicultiestherein have been encountered. Inasmuch as a high level of sulfateimpurities cannot be tolerated in either the feed for the diaphragmcells or the mercury cells, in the past, it has been necessary toprovide separate brine purification systems for both types of cells.Additionally, because of the detrimental effect of the chlorine whichremains in the recycle brine on cell operation, dechlorination of thisbrine prior to treatment and recycle to the mercury cell has also beenessential. It will be appreciated that the operation of separatepurification facilities for both the diaphragm cell and mercury cellbrine feed is expensive and tends to eliminate any cost advantageobtained by utilizing the solid salt recovered from the diaphragm cellto resaturate the mercury cell brine.

It is, therefore, an object of the present invention to provide anintegrated brine supply and purification system for use in aninstallation utilizing both mercury cells and diaphragm cells, wherebythe need of a separate purification system for the mercury cell brinesupply is eliminated.

Another object of the present invention is to provide an integratedsystem as described above in which the need for dechlorination of themercury cell brine prior to resaturation for recycle to the mercury cellis eliminated.

A still further object of the present invention is to provide anintegrated system as described above, which system will be flexibleenough to meet any change in operational demands of either the diaphragmcells or the mercury cells.

These and other objects will become apparent to those skilled in the artfrom the description of the invention which follows:

The drawing, which is attached hereto and forms a part hereof, is aschematic flo-W diagram illustrating one embodiment of the integratedbrine supply system of the present invention.

In the description of the invention and the claims which follow, theterms alkali metal and halide are intended to refer, respectively, tosodium, potassium, lithium, cesium, and rubidium and to the fluorides,chlorides, bromides and iodides. Additionally, the term alkali metal isalso meant to include barium, which in this environment, has theproperties of an alkali metal. However, because of its low cost andready availability, sodium chloride is the preferred alkali metal halideand for this reason, primary reference will be made hereinafter tosodium chloride brines.

In the process for the coordinated operation of diaphragm and mercurycells wherein solid salt recovered from the evaporation of the diaphragmcell catholyte or cell liquor is used to resaturate the depleted brinein the mercury cells, the improved method of the present inventionenvisions evaporating the diaphragm cell catholyte or cell liquor to analkali metal hydroxide concentration not substantially in excess of 35%,further evaporating the liquor to an alkali metal hydroxideconcentration of about 50%, recovering precipitated solid alkali metalhalide from each evaporation, utilizing only the recovered alkali metalhalide from the first evaporation to resaturate the depleted brine inthe mercury cell and purging a portion of the thus resaturated mercurycell brine feed to the diaphragm cell brine feed, the amount of saidpurged portion being sufiicient to maintain the impurities in themercury cell brine feed at a level which can be tolerated in the mercurycell.

It has been found that the alkali metal halide precipitated in theevaporation of the diaphragm cell catholyte to a concentration notsubstantially in excess of 35% alkali metal hydroxide is significantlylower in sulfate impurities than that precipitated in evaporating thecatholyte from about 35% to 50% alkali metal hydroxide concentration. Byusing this first precipitated alkali metal halide, there is obtained asubstantially pure, solid alkali metal halide for resaturating themercury cell brine feed. It has been further found that by purging aportion of the mercury cell brine feed back into the diaphragm cellbrine feed, which portion is sufficient to maintain the sulfateimpurities in the mercury cell at a tolerable level, other undesirableimpurities in the mercury cell brine feed are likewise maintained withintolerable limits. Thus, the needfor treating this brine, to removeimpurities, is eliminated and hence it is not necessary to dechlorinatethe brine.

More specifically, in the present method, a brine containing about 295g./l. NaCl is purified, particularly with respect to calcium andmagnesium impurities. This purification can desirably be effected byadding to the brine solutions of caustic soda and soda ash and/or sodiumbicarbonate, whereby the undesirable calcium and magnesium ions areprecipitated as the insoluble carbonates and hydroxides, respectively.The thus-treated brine is then settled and filtered to remove theprecipitated impurities and the sodium chloride concentration of thebrine is increased to between about 318 to 325 g./l. by adding solidsalt thereto. The pH of the brine is adjusted so as to be notsubstantially in excess of 10.2 by the addition of hydrochloric acid.The brine is then passed into the diaphragm cell wherein it iselectrolyzed to produce chlorine gas, which is given off at the anode,and the cell liquor containing about 11% sodium hydroxide and 14% to 15%sodium chloride, which cell liquor is removed at the diaphragm cellcathode.

As the cell liquor is recovered from the cathode compartment of thediaphragm cell, it is introduced into a multi-stage evaporator, whereinit is evaporated to a sodium hydroxide concentration not substantiallyin excess of about 35%. At this sodium hydroxide concentration, solidsodium chloride precipitates from the solution and is removed in anyconvenient manner. The liquor is then further evaporated to a sodiumhydroxide concentration of about 50%, during which evaporationadditional solid sodium chloride is precipitated and removed. Thislatter sodium chloride, from the 50% sodium hydroxide concentration, iseither discarded or recycled into the diaphragm cell brine feed streamto be used in the saturation of the raw brine from the brine wells. Asthis solid salt contains a high percentage of sodium sulfate, andadditionally, has a relatively high sodium chloride to sodium sulfateWeight ratio, i.e., about 4 to 5 parts by weight sodium chloride to 1sodium sulfate, it is preferably discarded rather than reusing it in thediaphragm process, inasmuch as this can be done without loss ofsubstantial quantities of sodium chloride. Alternatively, where saltcosts are high, it can be further concentrated and treated so as toremove substantially all of the sulfate and then reused.

The solid salt from the first evaporation of the diaphragm cellcatholyte is slurried with brine and a portion of this slurry isseparated and returned to the brine feed system of the diaphragm cells,wherein it is used to resaturate the brine to increase its sodiumchloride concentration from 295 g./l. to the desired 318 to 325 g./l.The remaining portion of the reslurried solid sodium chloride is passedinto a separation apparatus, wherein the solid and liquid portions ofthe slurry are separated and the liquid portion returned to thediaphragmcell brine steam.

The solid portion of the slurry from the separator is added to therecycled brine stream from the mercury cell, the sodium chlorideconcentration of which brine stream, in passing through the mercurycell, has been depleted to about 280 g./l. from the desired 305 to 310g./l. re quired for mercury cell operation. Suflicient of the solid saltrecovered from the separator is added to the recycled brine stream untilthe desired concentration of about 305 to 310 g./l. is achieved and thepH of the brine is then adjusted to about 4.5 to about 5.5 by theaddition of hydrochloric acid. After resaturation and prior to beingreturned to the mercury cell, a portion of the brine stream is purgedback to the diaphragm cell brine system wherein it is added to the brinefeed stream. The amount of this purge is sufiicient to maintain thesulfate impurities in the mercury cell brine system at not substantiallyin excess of 10.0 g./l.

The remainder of the brine, at a sodium chloride concentration of 305 to310 g./ 1., is passed into the mercury cell wherein it is electrolyzedto produce chlorine at the anode and a sodium amalgam at the mercurycathode, from which sodium hydroxide is recovered. In passing throughthe mercury cell, the sodium chloride concentration of the brine isdepleted to about 280 g./l. so that the brine is recycled to thesaturators wherein the sodium chloride concentration is increased by theaddition of solid salt obtained from the evaporation of the cell liquorfrom the diaphragm cell, before being returned to the mercury cell.

It will be noted that in the method as described above, only one brinepurification is required in order to obtain a brine feed of suitablepurity for both the diaphragm and mercury cells. Moreover, by purging aportion of the mercury cell brine feed back into the brine feed for thediaphragm cell, the sulfate and other impurities in the mercury cellfeed are prevented from building up to an intolerable level, thuseliminating the necessity for a separate purification system for themercury cell brine and hence the need for dechlorination of this brine.

Referring now to the drawing, as shown in the schematic flow diagram,raw sodium chloride brine from any convenient source, such as a brinewell or reservoir (not shown) is passed through purification apparatuswherein the raw brine is purified, particularly with respect to calciumand magnesium impurities. Inasmuch as the precise mechanism ofpurification does not form a part of the present invention, no detailsof the purification step have been shown on the drawing. Suflice it tosay that the calcium and magnesium impurities may conveniently beremoved from the raw brine by adding thereto solutions containing sodiumhydroxide and sodium carbonate, which materials cause the precipitationof these impurities so that they may be removed from the brine bysettling and filtration. After the impurities have been removed from theraw brine, it passes into a mixing tank wherein hydrochloric acid isadded to adjust the brine pH so as to not be substantially in excess ofabout 10.2. The brine is then saturated with solid sodium chloride tobring the sodium chloride concentration to within the range of 318 to325 g./l., which sodium chloride concentration, is desired for theoperation of the diaphragm cell. As will be explained in more detailhereinafter, the solid sodium chloride with which the purified brine issaturated may conveniently be a portion of that recovered from thediaphragm cell liquor.

The purified, saturated brine, having a sodium chloride concentrationbetween 318 and 328 g./l., passes into a diaphragm cell wherein it iselectrolyzed, chlorine gas being recovered at the anode and the cellliquor containing sodium hydroxide and sodium chloride being recoveredat the cathode. This cell liquor is passed into a multi-stage evaporatorwherein it is first evaporated to a sodium hydroxide concentration ofabout 35%, during which evaporation solid sodium chloride isprecipitated from the cell liquor. The liquor having a concentration ofabout 35% sodium hydroxide is then further evaporated to a sodiumhydroxide concentration of about 50%, during which evaporationadditional solid sodium chloride is precipitated. This latterprecipitation of sodium chloride, being high in sulfate impurities, ispreferably discarded as waste material.

The solid sodium chloride precipitated in evaporating the cell liquor toa sodium hydroxide concentration of about 35%, passes to a repulpingtank wherein brine is added so as to form a solid slurry of the sodiumchloride. After adjusting the pH of this slurry to not substantially inexcess of about 10.2, by the addition of hydrochloric acid, a portion ofthe slurry is returned to the diaphragm cell brine stream to be used insaturating the purified diaphragm cell brine to obtain the desiredsodium chloride concentration of about 318 to 325 g./l. From therepulping tank, the remainder of the sodium chloride slurry passes intoa separator wherein the liquid and solid portion of the slurry areseparated. The liquid portion, which has a sodium chloride concentrationof about 318 g./l., is returned to the purification portion of thediaphragm cell system to be used in making up the diaphragm cell feedstream.

The solid sodium chloride from the separator is then acidified with thehydrochloric acid to a pH within the rane of about 4.5 to 5.5 and passedinto a saturator wherein it is contacted with the depleted brinerecycled from the mercury cell. This depleted brine, which has a sodiumchloride concentration of about 280 g./l., is saturated with the solidsodium chloride until a brine is obtained having a sodium chlorideconcentration of about 305 to 310 g./l., which sodium chlorideconcentration is desired in the operation of the mercury cell. Thisbrine at the desired sodium chloride concentration, is then passed intothe mercury cell, wherein it is electrolyzed to form chlorine gas, whichis given off at the cell anode and sodium which combines with theflowing mercury film cathode to form a sodium amalgam. The sodiumamalgam is directed into a denuder wherein the mercury is recovered andrecycled to the cell and sodium hydroxide is formed and recovered as thesecond product of the electrolysis. As has been pointed out above, thebrine in passing through the mercury cell, is depleted to a point atwhich the sodium chloride concentration is only about 280 g./l. Inasmuchas this sodium chloride concentration is insuflicient for the operationof the mercury cell, after passing through the cell, the brine isrecycled to the saturator so that the sodium chloride concentration canbe increased to the desired level before the brine is returned to themercury cell for electrolysis.

It will be appreciated that, in essence, the mercury cell brine systemis a closed system, i.e., the brine passes through the cell and thesodium chloride concentration thereof is depleted, whereupon the brineis resaturated and then returned to the cell. In such a system,obviously, there will be a gradual build-up of impurities in the brine,such as sulfate, magnesium, and heavy metal impurities. To prevent theseimpurities from building up to an intolerable level, a stream of theresaturated brine is continuously removed and returned to the brinemixing tank in the diaphragm cell system. The impurities in this purgedstream of brine are not sufiicient to raise the impurity level in thediaphragm cell feed stream above that which can be tolerated.

It will, thus, be appreciated that the amount of brine which is purgedfrom the mercury cell brine stream will depend upon the impurities whichcan be tolerated in both the mercury cell and diaphragm cell brinefeeds. Therefore, the purged stream must be sufliciently large tomaintain the mercury cell brine impurities at a tolerable level, but itmust not be so large as to increase the impurities in the diaphragm cellbrine stream to a level which cannot be tolerated.

It has been found, that by recycling a portion of the mercury cell brinefeed at such a rate that the sulfate impurities are maintained at notsubstantially in excess of about 10.0 g./l., the other impuritiestherein, which are detrimental to mercury cell operation, will likewisebe maintained at a tolerable level, without increasing the sodiumsulfate content in the diaphragm cell brine feed above the upper limitof about 5.0 g./l. Inasmuch as the precise amount of this purge will,obviously, depend upon the rate of brine flow to both mercury anddiaphragm cells, which factors will vary considerably depending upon thedemands for chlorine and sodium hydroxide, no attempt will be made toset a precise rate of flow for this purge. It is believed that thoseskilled in the art can readily determine in each instance what this flowrate must be in order to maintain the sulfate impurities in the mercurycell brine at not substantially in excess of about 10.0 g./l., dependingupon variations of the above-mentioned factors.

In actual operation, raw sodium chloride brine from brine wells at asodium chloride concentration of 295 g./l. is pumped at the rate of 925gallons per minute, representing a total of 4,964 tons per day of water,1,640 tons per day of sodium chloride, 23.6 tons per day of calciumsulfate, and 0.315 ton of magnesium sulfate. This brine is then purifiedby adding thereto two streams of brine. The first brine stream at asodium chloride concentration of 219 g./l. flows at the rate of 58gallons per minute, representing a total of 314 tons per day of water,tons per day of sodium chloride and 21.1 tons per day of sodiumcarbonate. The second brine stream, which is a mixture from the mercurycell repulping tank and the overflow from the separator, at a sodiumchloride concentration of 315 g./l. is pumped at gallons per minute,representing a total of 1,013.4 tons per day of water, 408 tons per daysodium chloride, 13.40 tons per day sodium sulfate, 1.808 tons per daysodium hydroxide and 1.534 tons per day sodium carbonate. After thor- 7oughly admixing these two brine streams with the raw brine from the saltwells, the combined brine stream is reacted, settled and filtered toremove a total of 17.4 tons per day calcium carbonate and 0.15 ton perday magnesium hydroxide.

From the filter the brine stream passes to a mixing tank at the rate of1,178 gallons per minute. The sodium chloride concentration in thisbrine stream is 301 g./l. This brine stream represents a total of6,291.4 tons per day of water, 2,128 tons per day of sodium chloride,1.599 tons per day of sodium hydroxide, 4.244 tons per day sodiumcarbonate, and 38.41 tons per day sodium sulfate. In the mixing tank, tothe above is added the purged brine stream from the mercury cell brinefeed system, which purged stream is pumped at a rate of 124.5 gallonsper minute, representing a total of 655.6 tons per day of water, 231tons per day sodium chloride, and 7.40 tons per day of sodium sulfate.

From the mixing tank, two brine streams are taken off, the first goingto the resaturation tank for the diaphragm cell feed system and thesecond going to the mercury cell repulping tank. The first stream, tothe resaturati-on tank, at a sodium chloride concentration of 301 g./l.,flows at the rate of 1.068 gallons per minute, representing a total of5,687 tons per day of water, 1,932 tons per day sodium chloride, 37.51tons per day sodium sulfate, 3.474 tons per day sodium carbonate and0.33 ton per day sodium hydroxide. The second stream to the mercury cellsolid salt repulping tank, at a sodium chloride concentration of 301g./l., fiows at the rate of 235 gallons per minute, representing a totalof 1,260 tons per day of Water, 427 tons per day sodium chloride, 8.30tons per day sodium sulfate, 0.77 ton per day sodium carbonate and 0.07ton per day sodium hydroxide.

Within the diaphragm cell resaturation tank, the brine stream from themixing tank is combined with a salt slurry from the mercury cellrepulping tank, which stream is pumped at the rate of 49 gallons perminue, representing a total of 206 tons per day of water, 193 tons perday sodium chloride, 3.75 tons per day sodium sulfate, 0.40 ton per daysodium carbonate and 0.04 ton per day sodium hydroxide. From theresaturation tank the brine is pumped into the diaphragm cells at aconcentration of 318 g./l. sodium chloride at the rate of 1,117 gallonsper minute, representing a total of 5,893 tons per day water, 2,125 tonsper day sodium chloride, 41.26 tons per day sodium sulfate, 3.874 tonsper day sodium carbonate and 0.37 ton per day sodium hydroxide.

From the diaphragm cell, the cell liquor or catholyte, at a sodiumhydroxide concentration of about 135 g./l. and a sodium chlorideconcentration of about 210 g./l., is recovered and sent to theevaporator. Within the evaporator, the catholyte is evaporated to asodium hydroxide concentration of about 35%, during which evaporation, alow sodium sulfate content sodium chloride is recovered. The liquor isthen evaporated to a sodium hydroxide concentration of about 50%, duringwhich evaporation a high sodium sulfate content sodium chloride isrecovered. About 90% to 95% of the salt is recovered in the firstevaporation and about to 10% is recovered in the second. The highsulfate salt from the second evaporation may be discarded or furtherprocessed to remove the sulfate and recover the salt, while the lowsulfate salt from the first evaporation is directed to the mercury cellrepulping tank.

From the diaphragm cell caustic evaporator to the mercury cell repulpingtank is pumped 45 tons per day of water, 811 tons per day of sodiumchloride, 16.25 tons per day of sodium sulfate, 1.68 tons per day ofsodium carbonate and 3.258 tons per day of sodium hydroxide. To thissubstantially solid sodium chloride is added to the brine stream fromthe diaphragm cell mix tank as described above. Additionally, to therepulping tank are added 25.3 gallons per minute of water, representing151.6 tons per day of water and the overflow at the rate of 131 gallonsper minute, having a sodium chloride concentration of 318 g./l.,representing a total of 694.3 tons per day of water, 249.5 tons per daysodium chloride, 10.12 tons per day sodium sulfate, 1.23 tons per daysodium carbonate and 1.38 tons per day sodium hydroxide.

From the repulping tank three streams are taken, one of which goes tothe diaphragm cell brine purification, the second of which goes to thediaphragm cell brine saturation tank, both as described above, and thethird going to the separator at the rate of 220 gallons per minute,representing a total of 931.5 tons per day of water, 886.5 tons per dayof sodium chloride, 17.52 tons per day sodium sulfate, 1.75 tons per daysodium carbonate and 2.38 tons .per day of sodium hydroxide. In additionto this latter stream, 71.4 gallons per minute of water, representing444.6 tons per day of water, are also added to the separator.

The overflow from the separator is sent to the diaphragm cellpurification tank while the underflow stream at the rate of 162 allonsper mintue, representing a total of 684.6 tons per day of water, 637tons per day of sodium chloride, and 7.40 tons per day sodium sulfategoes to the mercury cell saturators. Within the mercury cell saturatorsthis stream is admixed with the depleted brine from the mercury cell,which brine has a sodium chloride concentration of 280 g./l. and ispumped into the saturators at the rate of 1,570 gallons per minute,representing a total of 8,451 tons per day of water and 2,640 tons perday of sodium chloride. From the saturators, a brine stream having asodium chloride concentration of 318 g./l. is pumped at the rate of1,714 gallons per minute, representing a total of 9,135.6 tons per dayof water and 3,277 tons per day of sodium chloride. To this stream isadded depleted brine from the mercury cell at a sodium chlorideconcentration of 280 g./l. and a flow rate of 280 gallons per minute,representing a total of 1,520 tons per day of 'water and 478 tons perday of sodium chloride. The combination of these two brine streams givesa brine having a sodium chloride concentration of 310 g./l., which brinestream is pumped to the unfiltered mercury cell brine storage at therate of 1,994 gallons per minute, representing a total of 10,6556 tonsper day of water and 3,755 tons per day of sodium chloride. a

This brine is then filtered and pumped to the mercury cell filteredbrine storage from where it is delivered to the mercury cell at a sodiumchloride concentration of 310 g./l., at the rate of 1,870 gallons perminute, representing a total of 10,000 tons per day of water and 3,524tons per day of sodium chloride. Aditionally, a portion of this filteredbrine is purged back into the diaphragm cell system, as described above,at the rate of 124.5 gallons per minute, representing a total of 655.6

. tons per day water, 231 tons per day sodium chloride and 7.40 tons perday sodium sulfate.

In operating the brine supply and purification system on both themercury and diaphragm cells, as described above, it is found that theimpurities in the mercury "cell brine feed no not build up to such alevel that the operation of the mercury cell is impaired, even thoughthere is no provision made for a separate purification of the mercurycell brine. Additionally, it is found that inasmuch as no purificationtreatment of the mercury cell brine feed is required no provisions need'be made for dechlorinating the mercury cell brine. It is, thus, seen bythe method of the present invention, that a brine supply andpurification system for use in the coordinated operation of diaphragmand mercury cells is provided, which system, by eliminating the need fora separate purification of the mercury. cell brine and thus the need fordechlorination thereof, is considerably less expensive to operate thanthose processes used in the prior art.

While there have been described various embodiments of the invention,the methods described are not intended to be understood as limiting thescope of the invention as it is realized that changes therewithin arepossible, and it is further intended that each element recited in any ofthe following claims is to be understood as referring to all equivalentelements for accomplishing substantially the same results insubstantiflly the same or equivalent manner, it being intended to coverthe invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. In the process for the coordinated operation of diaphragm and mercurycathode electrolytic cells for the electrolysis of alkali metal halidebrines, wherein solid salt recovered from the evaporation of thediaphragm cell catholyte is used to resaturate the depleted brine fromthe mercury cathode cells, the improvement which comprises evaporatingthe diaphragm cell catholyte to an alkali metal hydroxide concentrationof about 35%, recovering the precipitated solid alkali metal halideresulting from such evaporation, adding the thus-recovered solid alkalimetal halide to the depleted brine from themercury cell so as toresatu-rate said brine to the alkali metal halide concentration requiredfor mercury cathode cell operation and sending a portion of theresaturated mercury cathode cell brine back to the diaphragm cell brinefeed, the amount of said portion being sufiicient to maintain theimpurities in the mercury cathode cell brine feed at a level below thatwhich is detrimental to the operation of the mercury cell.

2. The method as claimed in claim 1 wherein the alkali metal halide isan alkali metal chloride.

-3. The method as claimed in claim 2 wherein the alkali metal chlorideis sodium chloride.

4. In the process for the coordinated operation of diaphragm and mercurycathode electrolytic cells for the electrolysis of alkali metal halidebrines, wherein solid alkali metal halides recovered from theevaporation of the diaphragm cell catholyte is used to resaturate thedepleted brine from the mercury cathode cells, the improvement whichcomprises evaporating the diaphragm cell catholyte to an alkali metalhydroxide concentration of about 35 further evaporating the resultingliquor to an alkali metal hydroxide concentration of about 50%,recovering the precipitated solid alkali metal halide resulting fromeach evaporation, adding only the solid alkali metal halide recoveredfrom the first evaporation of the catholyte to the depleted brine fromthe mercury cell, said quantity of solid alkali metal halide added beingsufiicient to raise the sodium chloride concentration of the depletedbrine to the level required for mercury cathode cell operation, andsending a portion of the thusresaturated mercury cathode cell brine backto the diaphragm cell brine feed system, the amount of said portionbeing Sufficient to maintain the sulfate impurities in the mercurycathode cell brine feed below about 10 g./l.

5. The method as claimed in claim 4 wherein the alkali metal halide isan alkali metal chloride.

-6. The method as claimed in claim 5 wherein the alkali metal chlorideis sodium chloride.

7. In the process for the coordinated operation of diaphragm and mercurycathode electrolytic cells for the electrolysis of alkali metal chloridebrine wherein solid alkali metal chloride recovered from the evaporationof the diaphragm cell catholyte is used to resaturate the depleted brinefrom the mercury cathode cells, an improvement which comprisesevaporating the diaphragm cell catholyte to an alkali metal hydroxideconcentration of about 35 further evaporating the resulting liquor to analkali metal hydroxide concentration of 50%, recovering the precipitatedsolid alkali metal chloride resulting from each evaporation, adding onlythe solid alkali metal chloride recovered from the first evaporation tothe depleted brine from the mercury cathode cell so as to resaturatethis brine to an alkali metal chloride concentration of about 305 to 310g./l., and sending a portion of the thus-resatura-ted mercury cell brineback to the diaphragm cell brine feed, the amount of said purged portionbeing sufiicient to maintain the sulfate impurities in the mercury cellbrine at a level of about 10 g./l.

8. The method as claimed in claim 7 wherein the alkali metal chloride issodium chloride.

9. A process for the coordinated operation of diaphragm and mercurycathode electrolytic cells wherein a sodium chloride brine iselectrolyzed to produce initially a sodium chloride-sodium hydroxidecatholyte solution, sodium amalgam, and chlorine respectively, whichcomprises resaturating that sodium chloride brine which is depleted insodium chloride content in passing through said mercury cathode cell,with solid sodium chloride which is recovered in evaporating thecatholyte of the diaphragm electrolytic cell, to a sodium hydroxideconcentration of about 35% by weight, the amount of said solid sodiumchloride added to said depleted brine being sufiicient to resaturate itto the sodium chloride concentration required for operation of themercury cathode cell, and sending a portion of the thus-re-saturatedmercury cell brine back into the brine feed system for the diaphragmcell from which the solid sodium chloride is recovered, the amount ofsaid portion being suflicient to maintain the impurities in the mercurycathode cell brine feed at a level below that which is detrimental tothe operation of said mercury cell.

References Cited in the file of this patent UNITED STATES PATENTS YugveIan. 1, 1929 Svanoe Dec. 9, 1958 OTHER REFERENCES

1. IN THE PROCESS FOR THE COORDINATED OPERATION OF DIAPHRAGM AND MERCURYCATHODE ELECTROLYTIC CELLS FOR THE ELECTROLYSIS OF ALKALI METAL HALIDEBRINES, WHEREIN SOLID SALT RECOVERED FROM THE EVAPORATION OF THEDIAPHRAGM CELL CATHOLYTE IS USED TO RESATURATE THE DEPLETED BRINE FROMTHE MERCURY CATHODE CELLS, THE IMPROVEMENT WHICH COMPRISES EVAPORATINGTHE DIAPHRAM CELL CATHOLYTE TO AN ALKALI METAL HYROXIDE CONCENTRATION OABOUT 35% RECOVERING THE PRECIPITATED SOLID ALKALI METAL HALIDERESULTING FROM SUCH EVAPORATION, ADDING THE THUS-RECOVERED SOLID ALKALIMETAL HALIDE SO THE DEPELETED BRINE FROM THE MERCURY CELL SO AS TORESATURATED SAID BRINE TO THE ALKALI METAL HALIDE CONCENTRATION REQUIREDFOR MERCURY CATHODE CELL OPERATION AND SENDING A PORTION OF THERESATURATED MERCURY CATHODE CELL BRINE BACK TO THE DIAPHRAGM CELL BRINEFEED, THE AMOUNT OF SAID PORTION BEING SUFFICIENT TO MAINTAIN THEIMPURITIES IN THE MERCURY CAHTODE CELL BRINE FEED AT A LEVEL BELOW THATWHICH IS DETRIMENTAL TO THE OPERATION OF THE MERCURY CELL.